As part of a continuing series of studies of neuronal specificity and the development and connections in the hippocampal formation, we propose to examine the organization of the efferent, commissural and associational connections of the hippocampus in the brain of a convenient primate, the squirrel monkey (Saimiri sciuresis), and one of the most interesting neurological mutants, the "reeler mouse", using principally the autoradiographic method for the anterograde tracing of neural pathways and the retrograde transport of horseradish peroxidase. The results of these studies on the monkey hippocampus should be of direct relevance for our understanding of the organization of the limbic system in the human brain, while the studies of the reeler mouse may throw some light on the mechanism responsible for the assembly of cortical structures. Concurrent with this we shall re-examine the efferent connections of the principal nuclei of the hypothalamus in the rat, cat and squirrel monkey; the development of the autoradiographic method for tracing pathways should make it possible, for the first time, to examine these connections in a way which is uncomplicated by the involvement of the various fiber systems that pass through, and adjoin, the relevant hypothalamic nuclei. We also plan to continue our in vitro analysis of the growth and maturation of isolated hippocampal pyramidal cells, with special reference to those surface properties which may be implicated in the genesis of the highly specific pattern of connections which these cells display.